What will happen if I will purchase a plane tickets being under 18?
If you mislead the airline and purchase the ticket yourself by claiming you're an adult, most likely nothing will happen. The airline may cancel the ticket though if they find out. See these somewhat similar examples for a 17 year old who opened accounts with Paypal, eBay, Amazon, Aliexpress.com that require you to be 18 and over.
So I think there's no point in sending money to parents since it will
do nothing. So the whole 18 years requirement seems useless to me.
That's not correct. Letting your parents purchase it removes that small probability of the airline voiding a contract they entered into with a minor, had you done the purchasing directly.
The issue is one about a financial transaction with a minor. A contract with a minor is voidable, basically you the minor can cancel it any time before execution.
A voidable contract is a valid contract and can
be enforced. Usually only one party is bound to the contract terms in
a voidable contract. The unbound party is allowed to cancel the
contract, which makes the contract void.
Basically up until any time the plane takes off, you (because you are the unbound party) can just tell the airline, I don't want to fly any more, give me my money back. And because you were a minor, you would be entitled to getting your money back. The airline doesn't want that kind of uncertainty.
So coming to it, the issue is not one about you flying as an unaccompanied minor. Many airlines allow children over 12 to fly as unaccompanied minors. In fact I just purchased a ticket last week for my 13 year old nephew to fly unaccompanied all the way to Africa.
Thus sending the money to your parents for them to purchase is materially and legally different from you doing the purchasing. If your parents purchase the ticket, the contract is binding and no longer voidable just because you're a minor.